cooled by radiation through the cast-iron foundation pan, while at Freiberg and Przibram oxidation is accelerated by means of an air blast.

At Freiberg the charge is 21 tons, which takes thirty-six hours to soften completely, and yields 85 per cent. by weight of softened lead. Three kinds of skimmings are taken off, which are sometimes divided into six classes as one variety runs into the next. The first skimmings after the melting down are of a dark yellow colour, contain most of the tin, and are, on that account, powdery. They consist mostly of antimoniate of tin and stannate of lead, but as oxidation proceeds antimoniates and arseniates of lead are found and the skimmings become pasty. All these skimmings (abstrich) are drawn near the door nearest the flue and are called "tin skimmings." Fumes of arsenic and antimony then appear, and the former element (together with some of the latter) oxidises rapidly to a light brown arseniate of lead, which is drawn off with a long iron hoe having a wooden blade and called "arsenic skimmings." The brown arseniate of lead gradually gives place to the almost black antimoniate and the next lot of skimmings is called "antimony skimmings." As the antimony becomes eliminated the black is more and more replaced by greenish-yellow litharge.

So long as there is any perceptible amount of antimony in the lead, a sample in a ladle will show globules of black skimmings revolving on the unoxidised surface of the metal like grease on water; but when nearly all the antimony is "out" the globules are replaced by a uniform yellow coating of litharge which forms quickly. Poured into a mould and skimmed just before solidification the bar shows the characteristic even, smooth, blue surface and lustre of soft lead.

At Przibram the charge is 22 tons, which is softened in twenty-four hours with a consumption of 9 per cent. of coal, and yields 81 per cent. of its weight as softened lead.

At modern works in the United States and England, it is customary to use water-jacketed softening furnaces built into large wrought-iron pans in order to reduce the amount of corrosion of the walls by lead antimoniate and oxide, which is very serious in the case of unjacketed furnaces. Some furnaces have been built with two complete pans, having a 3 to 4 inches space between, filled with water; but the cooling effect is then very great and causes excessive consumption of fuel to soften the lead sufficiently quickly. The corrosion of the hearth is never very serious and air cooling is quite sufficient; while the corrosion of the walls when water jacketed is very easily repaired with common clay or with brasque which is tamped in between charges.

*

Common clay is much better than fireclay for almost all internal "patch" repairs of furnaces of every kind.

Furnace Construction. The construction of a 35-ton softening furnace at the Port Pirie Works of the Broken Hill Proprietary Co., divested of its binders and water connections for simplicity, is shown in Figs. 111 to 115, in which a is the hearth separated from the firebox, b, by an air space beneath the bridge, which is supported by a massive casting as shown. The hearth is wholly enclosed in a tank of-inch mild-steel plate, to the upper part of which is attached the mild-steel water

Figs. 111 and 112.-Port Pirie Softening Furnace.

jacket of wedge section, detail of which is given in Fig. 114. The tank containing the hearth is supported upon a layer of 80lb. rails pretty close together, as shown. The water jacket has 14-inch water connections, and handholes here and there for cleaning. The position of the taphole is not shown, but Fig. 115 gives detail of the cast-steel tapping jacket, the taphole in which is closed by a screw plug. In some of the furnaces (the so-called "copper furnaces") the tapping jacket is placed at the side, while in others (the so-called "antimony furnaces") it is placed at the end, its exact position being determined by

the door frames form an integral part; and the furnace is also strongly braced with old rails and tie-rods to resist the expansion. The furnace gases pass through the opening into a

The whole

the most convenient arrangement of the works. upper part of the furnace above the steel-plate tank is enclosed in a close fitting casing of massive cast-iron plates, of which

ค

Figs. 113 and 114.-Port Pirie Softening Furnace.

horizontal flue, which crosses the end of the furnace and leads into an independent downtake for each furnace connected with the main underground flue of the refinery. There are three doors on each side of the furnace which serve for charging and skimming respectively, and one skimming door in the end opposite the firebox. The hearth is built of the best firebrick as an inverted arch, and must be laid with the closest possible fireclay joints to make it lead tight; the working bottom is similarly built as shown in Fig. 113, and tightly wedged against its skewbacks in the side walls. Air is frequently admitted through the bridge and roof to accelerate oxidation, and sometimes also a blast is used under the grate when inferior slack is to be used as fuel.

American and English softening furnaces are very similar, except that in the latter a simple water pipe extending round the furnace inside the iron or steel tank is sometimes used instead of a jacket; and that in the former the water jacket is frequently a rectangular box, the inner side of which is formed simply by the side of the wrought-iron hearth tank, while the top and bottom are of square bar or of channel iron rivetted to it and to the outer plate. Figs. 116 to 119 represent an American softening furnace with rectangular water jacket, and end taphole passing through an extension of the wrought-iron jacket. Other details of the furnace are, in the main, similar to those of the Australian furnace previously figured.

Method of Working.-The pigs of lead must be introduced carefully so as not to injure the working bottom, and within a period of from one and a-half to five hours after the charging is finished (according to the temperature employed) they are all melted and the dross ready for skimming. To assist in collecting the dross, a little fine coal is sometimes stirred in, and then the